class SingleDog {
  
  constructor() {}
  show() {
    console.log('我是个单例对象')
  }

}

SingleDog.getInstance = (function () {
  let instance = null
  return function() {
    if (!instance) {
      instance = new SingleDog()
    }
    return instance
  }
})()
     
const s1 = SingleDog.getInstance()
const s2 = SingleDog.getInstance()
console.log(s1 === s2);




